Interaction of the DNA-binding antitumor antibiotics, chromomycin and mithramycin with erythroid spectrin

The aureolic acid group of antitumor antibiotics, chromomycin A3 and mithramycin, are well established as transcription inhibitors, which bind reversibly to DNA at and above physiological pH, in the presence of divalent metal ions such as Mg2+. As part of our broad objective to elucidate their intracellular mode of action, other than association with DNA, we studied their interactions with the erythrocyte cytoskeletal protein, spectrin, in the absence and presence of magnesium. Different spectroscopic studies, such as absorbance, fluorescence and CD, have shown that both free chromomycin and mithramycin and their Mg2+ complexes bind to spectrin with an affinity higher than that reported for DNA. The affinity constants for the association of chromomycin and mithramycin (or their Mg2+ complexes) with spectrin are comparable with those for the association of spectrin with other cytoskeletal proteins, for example F-actin, ankyrin, protein 4.1, etc. The nature of the binding of the two antibiotics to spectrin is different. The mode of binding of the antibiotics with spectrin also changes in the presence of Mg2+. The interaction leads to a change in the tertiary structure of the protein. The relevance of the results to our understanding of the mode of action of the antibiotics is discussed.